The present invention relates to a comb filter for separating a color signal and a luminance signal from a color image signal, and more specifically, to an improvement of a color comb filter for removing a cross color and a luminance comb filter for removing noises in reproducing.
There will be described a conventional comb filter for extracting a luminance signal and a color signal from an image signal with reference to FIG. 1.
Numerals 1-4 shown in FIG. 1 denote first and second input terminals and first and second output terminals, respectively.
In FIG. 1, an adder 5 superposes a reproduced luminance signal S.sub.a through the first input terminal 1 and a reproduced chrominance signal S.sub.b through the second input terminal 2, both of which are demodulated by a video tape recorder (VTR) or a television receiver which is not shown in the figure to generate an image signal S.sub.c. The image signal S.sub.c is supplied through a charge coupled device (CCD) delay element 6 and a low pass filter 7 to one of input terminals of a subtracter 8 as a delayed image signal S.sub.d. The low pass filter 7 removes a CCD clock component included in the delayed image signal. The image signal S.sub.c is supplied to the other terminal of the subtracter 8 to obtain a chrominance signal S.sub.e as an output thereof. The chrominance signal S.sub.e is supplied to a band pass filter 9 which removes non-correlative components from the chrominance signal S.sub.e to generate a processed color signal S.sub.f. Furthermore, the chrominance signal S.sub.e is also supplied through a limiter 10 to an amplifier 11 which reduce an error by the non-correlation of the luminance signal, thereby outputting an amplified color signal S.sub.g to one of input terminals of a subtracter 12. The image signal S.sub.c is supplied to the other input terminal of the subtracter 12 for outputting a video signal S.sub.h of which noise components of the luminance signal is offset. The video signal S.sub.h is supplied to a low pass filter 13 which removes color signal components, thereby outputting a processed luminance signal S.sub.i. The processed luminance signal S.sub.i is outputted through the first output terminal 3, and the processed chrominance signal S.sub.f is outputted through the second output terminal 4.
Next, there is described a signal processing in the conventional example as shown in FIG. 1 in accordance with FIGS. 2A-2I which show signal waveforms of respective portions in the comb filter, respectively.
FIG. 2A shows a waveform of the reproduced luminance signal S.sub.a, and FIG. 2B shows a waveform of the reproduced chrominance signal S.sub.b. The noise components are included in the reproduced luminance signal S.sub.a. Both of the signals S.sub.a and S.sub.b are added with each other to be the image signal S.sub.c as shown in FIG. 2C. The image signal S.sub.c is delayed by the CCD delay element 6 for a 1H (a time corresponding to one horizontal scanning) to be the delayed image signal S.sub.d as shown in FIG. 2D. When the signals are used in a National Television System Committee (NTSC) color system, a phase of the color signal is inverted with 180 degrees every 1H. Accordingly, a subtraction between the image signal S.sub.c and the delayed image signal S.sub.d delaying for the 1H is shown by the chrominance signal S.sub.e in FIG. 2E because this subtraction represents an addition with respect to the color signal components. Since the comb filter separates the luminance signal and the chrominance signal on the basis that the correlation between adjacent scanning lines is high, a separation accuracy of both signals becomes lower in a portion of a non-correlation. Since the chrominance signal S.sub.e appears as an additional signal of a non-correlative component of the luminance signal (Y) and an output signal of the comb filter, in order to remove the non-correlative component of the luminance signal as shown in FIG. 2E, the additional signal is passed through the band pass filter 9 to generate the processed chrominance signal S.sub.f as shown in FIG. 2F.
Furthermore, the chrominance signal S.sub.e as the output of the subtracter 8 becomes the amplified color signal S.sub.g, as shown in FIG. 2G, by passing through the limiter 10 and the amplifier 11, and the subtraction between the amplified color signal S.sub.g and the image signal S.sub.c forms the video signal S.sub.h of which the noises components shown in FIG. 2H are offset. Since the correlation noises in the low frequency band of the luminance signal are removed by the subtraction from the video signal S.sub.h, it is possible to form a luminance signal output of the comb filter. However, the video signal S.sub.h has a remaining color signal which is passed through the limiter 10 and amplifier 11, this video signal S.sub.h becomes an output of the comb filter, which is formed by the color signal component superposed with the luminance signal. The color signal component is removed by the low pass filter so as to obtain the processed luminance signal S.sub.i shown in FIG. 2I.
Accordingly, the output signal of the luminance comb filter is always passed through the low pass filter 13, so that the this filter has the problem of causing a band width of the luminance signal to become narrow. Furthermore, the processed color signal S.sub.f is always passed through the band pass filter 9, thereby having the problem of causing a band width of the color signal to become narrow.
As described above, in the conventional comb filter which is considered with the non-correlation of the luminance signal, since the filter (the band pass filter 9 or the low pass filter 13) is contained in the main signal path through which the non-delayed image signal is always passed, it is unsuitable to reduce the band width of the processed color signal and the processed luminance signal.
In order to eliminate the above problem, there is proposed Japanese patent publication No. 5-30117 (1993) [Laid-open No. 3-113987 (1991)] as the prior art of the present invention. A color video signal reproducing circuit desclosed in this publication comprises, as shown in FIG. 3, an adder 5 for adding the reproduced luminance signal S.sub.a with the reproduced chrominance signal S.sub.b, a 1H delay line 6 for delaying an added signal S.sub.c for a one horizontal scanning period (1H), a subtracter 8, a band pass filter 9 for outputting the processed chrominance signal S.sub.f after passing through a band of the subtracted signal S.sub.e outputted from the subtracter 8, a low pass filter 7 for passing through a low frequency band of the subtracted signal S.sub.e, a limiter 10 for limiting the output of the low pass filter 7, and a subtracter 12 for outputting the processed luminance signal S.sub.i after subtracting the output of the limiter 10 from the reproduced luminance signal S.sub.a.
Even though the circuit shown in FIG. 3 does not include the filter circuit in the first main signal path M.sub.1, since the filter circuit is not omitted from the second main signal path M.sub.2, there is a problem that the band of the chrominance signal must be narrow.